The invention relates to an angle demodulator comprising a phase detection arrangement having a first input for applying a square-wave, angle-modulated input signal to be demodulated, whose modulation frequency is of the same order as the carrier frequency, a second input and an output, and also comprising a carrier device for applying a quadrature detection carrier to the second input of the phase detection arrangement, at a central frequency of the input signal said quadrature detection carrier being shifted at least substantially 90.degree. in phase relative to the input signal and a low-pass filter connected to the output of the phase detection arrangement.
An angle demodulator of this type may be constructed as an FM quadrature demodulator as is known, for example, from U.S. Pat. No. 4,426,735 or as a PLL-FM-PM demodulator as is known, for example, from U.S. Pat. No. 3,564,434.
In the case of an FM-quadrature demodulator the FM-modulated input signal to be demodulated is generally applied directly on the one hand and through a frequency-dependent phase shifter on the other hand to the phase detection arrangement. The frequency-dependent phase shifter realizes a frequency-dependent phase shift of the input signal, which is approximately 90.degree. at a central frequency, generally the frequency which is centrally located in the frequency spectrum of the input signal. The quadrature detection carrier thus obtained is multiplied by the input signal resulting in the baseband frequency modulation signal which is available for further signal processing at the signal output of the angle demodulator after selection in the low-pass filter.
In the case of a PLL-FM/PM demodulator the phase detection arrangement and the low-pass filter combined with a voltage-controlled oscillator constitute a phase-locked loop in which the voltage-controlled oscillator supplies said quadrature detection carrier to the phase detection arrangement. Due to the loop operation the frequency of the quadrature detection carrier follows that of the input signal with a phase difference of 90.degree.. The control signal for the voltage-controlled oscillator, i.e. the output signal from the low-pass filter thereby presents the baseband angle modulation signal.
In the multiplier circuit of the known angle demodulator a very strong second order interference product having twice the instantaneous frequency of the input signal is produced in addition to the desired baseband modulation signal. This second order interference product does not only occur in the case of multiplication of sinusoidal carriers, but also in the case of multiplication of non-sinusoidal, for example, amplitude-limited or squarewave carriers, which are often applied for practical reasons. In the latter case higher even order interference products may also be produced which, however, relative to the desired baseband modulation signal are further remote in frequency than said second order interference product. An acceptable interference suppression in the low-pass filter is only possible when the second order interference product (and hence also all higher order interference products) is sufficiently far remote in frequency from the desired baseband modulation signal.
In certain FM/PM signal processing devices and FM/PM transmission systems the difference between the minimum frequency of the input signal and the maximum modulation frequency is, however, chosen to be comparatively small and the modulation frequency is, at least instantaneously, of the same order as the carrier frequency. This may be necessary, for example, for video recorder (VCR's) and video record players (VLP's) to record or read information with a high packing density, for example, in FM-receivers with swing compression to make an integratable realization possible and for example in wireless telephone apparatus to comply with legal prescriptions. In that case said second harmonic interference may occur very close to or, if the modulation frequency is larger than twice the carrier frequency, it may even occur within the frequency range of the desired baseband modulation signal so that an adequate interference suppression by means of simple filters is not possible.